1. Field of the Invention
The present invention relates to semiconductor electronic integrated circuits and fabrication methods, and, more particularly, to integrated circuits made of III-V compound semiconductors including heterostructure devices such as heterojunction bipolar transistors.
2. Description of the Related Art
Semiconductor devices made of gallium arsenide (GaAs) are preferred over devices made of silicon for high frequency applications due to the higher mobility of carriers in gallium arsenide; however, gallium arsenide material and fabrication technology lag far behind that of silicon. For example, gallium arsenide metal-semiconductor field effect transistor (MESFET) integrated circuits with more than a 1,000 gates have been fabricated (see, Toyoda et al, A 42ps 2K-Gate GaAs Gate Array, 1985 ISSCC Dig.Tech.Papers 206), but the precise control of device parameters such as threshold voltage for larger scale integration has not yet been achieved. Similarly, high electron mobility transistors (HEMTs), which are MESFETs that use the two dimensional electron gas at a heterojunction such as at an interface of gallium arsenide and aluminum gallium arsenide, provide fast devices but suffer from the lack of precise control of device parameters; see Mimura et al, High Electron Mobility Transistors for LSI Circuits, 1983 IEDM Tech Digest 99.
Bipolar transistors have several advantages over MESFETs for high speed, large scale integration applications; for example, the turn-on voltage is determined by physical parameters and is not sensitive to the geometry and doping levels as is the threshold voltage of MESFETs or HEMTs. However, the fabrication of gallium arsenide bipolar (or heterojunction bipolar) transistors suitable for high speed applications, such as ECL, is complicated by the need for both a thin base and low base resistance. Indeed, as given by S.Sze, Physics of Semiconductor Devices .sctn. 3.3 (Wiley-Interscience, 2d. Ed. 1981), the maximum frequency of oscillation is: ##EQU1## where f.sub.T is the current-gain cutoff frequency, r.sub.b is the effective base resistance, and C.sub.c is the effective collector capacitance. The cutoff frequency, in turn, is related to the emitter-to-collector delay time .tau..sub.ec by ##EQU2## and .tau..sub.ec decreases with decreasing base thickness. Thus there is a need for a base that is simultaneously thin and of low resistance.
H. Kroemer, RCA Rev. 332 (September 1957), suggested a graded composition base, which has a graded bandgap to provide a built-in quasi-electric field through the base, to accelerate injected carriers in the base to the collector. This improves the high speed performance of a bipolar transistor but does not address the problem of lowering the base resistance for a given base thickness.
Furthermore, a thin base aggravates the problems of fabrication and high base contact resistance because an etch must stop at the base and not penetrate it. The usual etches for GaAs (wet etches such as hydrogen peroxide plus sulfuric acid in water and dry etches such as plasmas and reactive ion etching (RIE) with chlorine compounds like CCl.sub.2 F.sub.2 and BCl.sub.3) are not very selective with regard to doping type. Most wet chemical etches, such as hydrogen peroxide plus ammonium hydroxide, and RIE etches, such as Cl.sub.2 and CCl.sub.2 F.sub.2, etch GaAs faster than Al.sub.x Ga.sub.1-x As. In the fabrication of HBTs a selective etching of both GaAs and Al.sub.x Ga.sub.1-x As (with x.ltoreq.0.3) is required; and RIE with BCl.sub.3 has a weak AlAs mole fraction dependence in etching Al.sub.x Ga.sub.1-x As. See, for example, Cooperman et al, Reactive Ion Etching of GaAs and AlGaAs in a BCl.sub.3 -Ar Discharge, 7 J.Vac.Sci.Tech.B 41 (1989). Thus there is a problem of etching in fabrication of HBTs with Al.sub.x Ga.sub.1-x As.